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dc.rights.licenseRestricted to current Rensselaer faculty, staff and students. Access inquiries may be directed to the Rensselaer Libraries.
dc.contributorPicu, Catalin R.
dc.contributorBlanchet, Thierry A.
dc.contributorManiatty, Antoinette M.
dc.contributorShi, Yunfeng
dc.contributor.authorKhan, Mohammad Jane Alam
dc.date.accessioned2021-11-03T09:24:45Z
dc.date.available2021-11-03T09:24:45Z
dc.date.created2021-07-07T16:14:29Z
dc.date.issued2020-12
dc.identifier.urihttps://hdl.handle.net/20.500.13015/2672
dc.descriptionDecember 2020
dc.descriptionSchool of Engineering
dc.description.abstractCross-slip is also investigated, and it is determined that only screw dislocations with [100] Burgers vector may cross-slip. The estimated values of activation energy and activation volume are found to be small, suggesting frequent cross-slips among slip systems sharing the [100] Burgers vector. Cross-slip is not hindered by the application of high pressure, which is important for shock loading situations.
dc.description.abstractShear localization happens when dislocation motion cannot accommodate the high strain rates imposed in the crystal. Shear localization is observed to be more frequent in β-HMX at higher pressures when dislocation motion is hindered by steric shielding. This study evaluates the shear band viscosity as a function of the applied pressure, temperature, and shear strain rate using atomistic models of the HMX crystal. It is observed that the viscosity of a fully formed shear band decreases as a power function of the strain rate (shear thinning) and decreases linearly with increasing temperature. The non-Arrhenius dependence of the viscosity on temperature is due to the high strain rates involved and the high non-equilibrium conditions in which the band operates. Pressure increases the viscosity of the fully formed band significantly. The band behavior is observed to be independent of the crystallographic orientations. It is shown that viscosity can be expressed exclusively in terms of the density of the non-crystalline material in the band, and hence, the results can be explained in terms of the excess free volume theory previously developed for shear bands in metallic glasses. The stress required to nucleate a shear band from a straight pre-existing dislocation is also reported as a function of the applied pressure, temperature, and shear strain rate.
dc.description.abstractThis study investigates and reports dislocation properties related to strain hardening in the crystal. The dislocation line energy, core energy and line tension are evaluated based on theoretical and atomistic calculations. The dislocation line and core energies are approximately independent of the orientation of the dislocation line in the respective slip system. However, the line tension is strongly orientation-dependent. The strength of dislocation junctions is evaluated based on this information. It is observed that the junction strength for crystals with dislocation densities reported for HMX is low in comparison to the CRSS. This suggests that dislocation-dislocation interaction does not contribute significantly to strain hardening. Estimates of the critical stress for the operation of the Frank-Read source, cross-slip, and homogenous nucleation are reported.
dc.description.abstractCyclotetramethylene-tetranitramine (β-HMX) is a molecular crystal which is widely used as explosives and propellants because of its energetic nature, high energy release rate, and stability. Plastic deformation is one of the key mechanisms causing the initiation of the decomposition reaction leading to detonation. However, the mechanisms of plastic deformation in β-HMX are still unclear and are currently investigated extensively. This study targets the investigation of various plastic deformation modes aiming to develop an understanding of the mechanisms involved in the deformation of the crystal. The first step in this path is to evaluate the critical resolved shear stress (CRSS, i.e., the resolved stress required to move an isolated dislocation at finite temperatures) and the dislocation mobility at finite temperatures for the most glissile slip systems previously determined to be active in this material. The results reveal weak temperature dependence of CRSS and mobility, which is explained by the fact that the Peierls barrier for dislocation motion is high in all slip systems. The complex structure and anisotropy of the crystal render dislocation motion highly orientation dependent. Stable stacking faults are observed in certain crystallographic planes, and in these cases, dislocations move by splitting into partials.
dc.language.isoENG
dc.publisherRensselaer Polytechnic Institute, Troy, NY
dc.relation.ispartofRensselaer Theses and Dissertations Online Collection
dc.subjectMechanical engineering
dc.titleMechanisms of plastic deformation in molecular crystal cyclotetramethylene-tetranitramine (β-HMX)
dc.typeElectronic thesis
dc.typeThesis
dc.digitool.pid180513
dc.digitool.pid180514
dc.digitool.pid180515
dc.rights.holderThis electronic version is a licensed copy owned by Rensselaer Polytechnic Institute, Troy, NY. Copyright of original work retained by author.
dc.description.degreePhD
dc.relation.departmentDept. of Mechanical, Aerospace, and Nuclear Engineering


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